I'd try to move it earlier. Oct and going into first 2 weeks of Nov are better, IMO, for the very fact that programs are interviewing in Dec/Jan and if you get interviews in Dec, you don't want to leave your externship to interview. They're typically pretty understanding but if you're going to be absent for more than 1 or 2 interview's worth of time, your externship program director may look down on that. Personally I think Oct is THE perfect time because it's close enough to interview season where they will remember you.
Nov is ok too, but I would schedule the interview at the end of the externship so that you leave on a good note. But again, interview season is pretty much mid/late Nov going into Jan.